Here’s your magical template. Write “Introductions!” in the subject line. (Maybe don’t use an exclamation point if, for example, one of the two people in your introduction is a grief counselor, a lawyer, a drug lord, or an escort service. I don’t know what you get up to in your free time.) Then write, “[Name], meet [Other name]. [Name], [Other name] is the person I was telling you about who [is selling item/can help with thing/is new to your area/whatever].” Then, on the next line, on its own so it is VERY clear, write, “I’ll let you two chat!” Then sign the email with something like “Have a great day! [Your name]” You (should) have provided your acquaintances with sufficient evidence that you do not wish to be included in their email chain going forward. If they STILL reply all, email “ARE YOU KIDDING ME? Don’t talk to me! Don’t look at me!” Or just ignore it, ugh.